1. Field of the Invention
This invention relates to pump jacks, and more particularly to a pump jack capable of using a speed reducer of relatively small size.
2. Description of the Prior Art
Pump jacks are used in pumping underground liquids up to the surface. Pump jacks convert the circular motion of a motor into the reciprocal motion of a well rod. The well rod is the pump which extends underground down through the well head.
Previous pump jacks generally comprise in part a double pitman rod which is connected to a speed reducer through a crank. At the top and bottom of the stroke of the walking beam, the pitman rod is directly over the center of rotation of the cranks. Due to this alignment between the pitman rod and the center of rotation of the crank, the motion of the walking beam is such that there is a sudden jerk as the walking beam reaches the top and bottom of its stroke and reverses direction. This jerking results in great forces in the pitman rod and in the crank center shaft.
Since the crank center shaft is solely supported by the speed reducer, all the forces transferred from the pitman rod into the crank are also transferred into the speed reducer. Because of these large non-torsional forces the speed reducer is required to absorb during the motion of the walking beam, the speed reducer has to be relatively large and has a tendency to break down frequently. The relatively large speed reducers require a great deal of power to overcome their frictional losses and their internal inertial forces. These previous pump jacks require such large speed reducers that a large portion of the power requirement of the pump jack is that due to the friction and inertia inside the speed reducer.